The semi-conductor industry employs ion implantation in the manufacture and processing of very large scale integration (VLSI) microprocessors. Ion implantation is a process by which dopants are added to a semiconductor wafer by impacting energized and accelerated charged atoms or molecules (positive or negative ions) against semiconductor substrates. One of the objectives of ion implantation is to introduce a desired atomic species uniformly into a target material, the semiconductor wafer.
Measuring and/or attaining precise dose uniformity over a wafer surface is typically accomplished by scanning a beam of ions across the wafer surface. To ensure the surface of the wafer is “painted” uniformly by the beam generally requires feedback control of beam current, beam scan dwell, and the like to ensure acceptable across wafer implanted dose uniformity. Directly measuring beam current, for example while the ion beam is directly on the wafer is nearly impossible by conventional means and only indirect beam current monitoring methods are practical.
A Faraday cup is typically used in an ion implantation system as an indirect beam current measurement method. The Faraday cup system is utilized, for example, to determine the ion beam current, wherein the metal “cup” intercepts and traps the ionic particles. Typically, when the ion beam traverses off of the wafer it is measured with the Faraday cup, which is placed in the path of the ion beam. The ion beam charged particles strike the cup transferring charges from the beam to the cup and the resultant charge can be converted to an equivalent current indicative of the number of ions striking the cup. In this way the implantation current that the wafer “sees” can be determined as the beam is scanned back and forth across the wafer and can be adjusted if necessary. However, there are several problems associated with employing the approach, discussed supra. Depending on how fast the system scans the wafer will determine the speed at which the beam can be monitored, for example in some systems this is approximately every ten to twenty milliseconds. Even though the ion beam is measured at the Faraday cups, it is not known what is actually taking place on the wafer surface.
During an electrical glitch, for example, the ion beam is interrupted for a given time (e.g., one microsecond) that if taking place while the ion beam is “on wafer” would go undetected using a Faraday cup system.
An additional issue is that there are often background gases in the chamber, and when the ion beam strikes the background gas molecules the molecules can pull the charge off of the ions, neutralizing them. Subsequently, the “ion” keeps moving but it no longer has a charge (becomes a “fast atom”). The Faraday cup which detects charge no longer responds to or recognizes the fast atom, even though that atom can be implanted in the wafer and can change the wafer properties.
In view of the above problems it would therefore be desirable to have a system and method which mitigates such issues. Thus, there exists a need for an improved system and method for determining dose uniformity in semiconductor implantation.